The present invention relates to steering column torque sensors.
Power assisted steering is a standard motor vehicle equipment feature. It happens that in order for a typical power steering control system to properly operate, a steering column torque sensor must be included in the system to close the control loop. Torque sensors, such as resistance strip/strain gauge sensors, capacitance sensors, magneto-elastic sensors, and transformer/strain gauge sensors, have been provided to determine the torque on the steering column. However, these sensors lack the sensitivity required for many of the present power steering control systems. Moreover, these sensors are extremely sensitive to changes in temperature and have limited durability.
The present invention has recognized the above-mentioned prior art drawbacks, and has provided the below-disclosed solutions to one or more of the prior art deficiencies.
A sensor for measuring differential angular displacement between an upper shaft segment and a lower shaft segment includes an upper target wheel that is coupled to the upper shaft segment and a lower target wheel that is coupled to the lower shaft segment. Each target wheel forms a plurality of slots and the lower target wheel is coaxially aligned with the upper target wheel. Moreover, the sensor includes at least one excitation coil that is energizable to create an alternating current magnetic field that, in turn, generates eddy currents in the target wheels. The sensor also includes at least one receiver coil that receives an alternating current signal. The amplitude of the receiving signal is a function of the combination of the alternating current magnetic field generated by the excitation coil and the eddy current blocking effects of the target wheels. Accordingly, the receiver coil senses a change in the alternating current signal reaching the receiver coil when the target wheels move relative to each other, and the sensor outputs a signal representative of the differential angular orientation of the target wheels.
In a preferred embodiment, a housing surrounds the coils and the target wheels. Furthermore, a torsion bar mechanically couples the upper shaft and the lower shaft. Preferably, the number of slots formed by each target wheel is equal to each other. The housing defines a vertical axis and the target wheels are disposed within the housing such that the wheels are perpendicular to the axis.
Also, in a preferred embodiment, the sensor includes at least one reference coil that is coaxially aligned with the target wheels. Moreover, the sensor includes a reference target wheel which may be one or both of the upper and lower target wheels. As intended by the preferred embodiment, the reference coil and the reference target wheel are used to compensate for changes in the sensor caused by temperature changes and changes caused by axial misalignment of the target wheels relative to the coils.
In another aspect of the present invention, a sensor for measuring differential angular displacement between an upper shaft segment and a lower shaft segment includes a target wheel that forms a plurality of slots. In this aspect of the present invention, the slots are arranged such that they from plural concentric rings. Furthermore, a plurality of constant area slots are formed which are unaffected when the target wheels rotate relative to each other.
In yet another aspect of the present invention, a power steering control system includes a microprocessor, a power source, and a steering column differential angle position sensor. The steering column differential angle position sensor is electrically coupled to the microprocessor, electrically coupled to the power source and mechanically coupled to a steering column. In this aspect of the present invention, the differential angle position sensor transmits a signal to the microprocessor that represents a differential angular displacement between an upper target wheel and a lower target wheel. The signal varies based on slot alignments between the target wheels.
In still another aspect of the present invention, a method for controlling a power steering system includes installing an upper target wheel a first steering shaft segment and installing a lower target wheel on a second steering shaft segment. Each target wheel has plural slots. The method also includes determining a differential angular position between the upper target wheel and the lower target wheel based on an alignment of the slots.
In yet still another aspect of the present invention, a target wheel is used in a sensor for measuring differential angular displacement between an upper shaft segment and a lower shaft segment. The target wheel forms a plurality of concentric circular rows of slots. The rows alternately form relatively narrow slots and relatively wide slots. In this aspect of the present invention, the slots are somewhat rectangular and curved to match the periphery of the target wheel.
In yet another aspect of the present invention, the target wheels form a plurality of slots that form a constant area regardless of the different position of the target wheels. In this aspect a reference signal can pass through to a reference coil, forming the basis for thermal and mechanical corrections.